1. Field of the Invention
The present invention relates to a communication system, a communication apparatus and a communication method, which enable the transmission of only a partial number bit string that is a part of a number bit string to be transferred according to LSB encoding and the like, and which employs a sliding window method. In the sliding window method, an interpretation interval, which is a range of the partial number bit string decodable into the number bit string is sequentially slid.
2. Description of the Related Art
In a wireless communication system such as a mobile communication system, a transmitted packet and the like are frequently lost in a wireless section in comparison with a wired communication system. Accordingly, in general, an automatic retransmission control (for example, an ARQ) to retransmit the lost packet and the like is widely used (for example, refer to Japanese Patent Application Laid-open Publication No. 2002-9741 and “High Capacity-Spatial Division Multiple Access (HC-SDMA) WTSC-2005-032 (ATIS/ANSI)).
Moreover, in the wireless communication system, in order to efficiently utilize a wireless resource such as a frequency band, there are also established a variety of encoding methods of reducing the number of bits of information (compressing information) to be transferred. For example, LSB encoding is known. With the LSB encoding, a transmitter transmits information (partial number bit string) of only the defined number of least significant bits (LSB) form a number bit string (V_target) to be transmitted by the transmitter. The number bit string to be transmitted by the transmitter includes a sequence number used in a header of the packet, or the like.
Since the LSB encoding causes the information of only the defined number of bits from LSB to be transmitted, it is necessary for a receiver to add information of higher-order bits, which has not been transmitted, to the received information of the defined number of LSB, and to decode the received information into the number bit string (V_target).
Specifically, the transmitter and the receiver share a reference value (V_ref). The transmitter transmits the sequence number (SN) as a part of the number bit string to be transferred. When it is assumed that the number of bits of the sequence number (SN) is k bits, the receiver that shares the reference value (V_ref) receives the sequence number (SN), and thereby can interpret a value within a range of V_ref to V_ref+2k-1 as the V_target, that is, as the number bit string to be originally transmitted by the transmitter. Here, the range of V_ref to V_ref+2k-1 is referred to as an “interpretation interval”.
Moreover, in the case of using the LSB encoding, a sliding window method is also widely used in order to make it possible to shift the above-described interpretation interval. In the sliding window method, a variable (P) is introduced, and a value used as the reference value (V_ref) is sequentially slid. Specifically, when the partial number bit string is decoded into the number bit string (V_target) by using the sequence number received by the receiver, the reference value (V_ref) is updated in the transmitter and the receiver.
Incidentally, in the wireless communication system, in general, a packet size therein frequently becomes extremely small in comparison with that of the wired communication system. In other words, in order to enhance transfer efficiency of the information per packet, it is desirable that the number of bits assigned to the header of the packet be reduced as much as possible.
However, in the case where the sequence number (SN) encoded according to the LSB encoding is transferred as the header of the packet, and where the number of bits assigned to the header is reduced, the number of bits expressible as the sequence number (SN) is limited. Moreover, in the LSB encoding into which the above-described sliding window method is introduced, a value used as the reference value (V_ref) is sequentially slid during the communication, and this also sequentially slides the interpretation interval in which the receiver can correctly decode the partial number bit string into the number bit string (V_target) by using the sequence number (SN).
For this reason, with the LSB encoding into which the sliding window method is introduced, in the case where the number of bits expressible as the sequence number (SN) is limited, and where a packet that does not belong to the current interpretation interval is retransmitted in response to an ARQ, the receiver cannot correctly decode a partial number bit string into a number bit string (V_target) by using the retransmitted packet. This causes a problem of breaking synchronization of the reference value (V_ref) between the transmitter and the receiver.
Instead, in order to avoid an occurrence of the above-described problem, information indicating that the received packet is the retransmitted packet may be added to the header. However, this produces another problem that the number of bits to be assigned to the header is undesirably increased.